There is strict maintenance of the copy number of low-copy bacterial plasmids implying the existence of a strong control mechanism over such replication. Previous efforts involving the development of a stochastic model of the replication of [1 -< 2] plasmids in the bacterial cell cycle have suggested that plasmid replication control intensifies with the age of the plasmid. Fitting of the model to labelled-plasmid DNA per cell values, measured in cells subsequent to thymidine pulse-labelling and nitrocellulose membrane elution, has revealed a monotonically increasing transition probability for replication of F plasmids in accordance with theory. The model is now being refined and applied to data to determine whether the transition probability exhibits a similar dependence on age when slightly higher copy- number plasmids growing in host cells of different cycle times are considered. (This is a continuation of Intramural Research Project Z01-RR-10484-01 BEI.)